The invention relates to a structured catalyst including microporous oxides of silicon, aluminum and titanium, and a process for producing this catalyst.
Microporous aluminosilicates of the structural type MFI (ZSM-5) and MEL (ZSM-11), and methods for producing them by hydrothermal synthesis are known from U.S. Pat. No. 3,702,886 and DE 21 19 723. These patent documents are entirely incorporated herein by reference. The structural types MFI and MEL are also described in W. M. Meier and D. H. Olson, Atlas of Zeolite Structure Types, Sutterworth-Heinemann, 1992, which is also entirely incorporated herein by reference. Aluminum-free materials with identical structures, known as silicalite-1 and silicalite-2, are disclosed in U.S. Pat. No. 4,073,865; U.S. Pat. No. 4,061,724; D. M. Bibby, N. B. Milestone, and L. P. Aldridge, Nature 280, 664 (1979); and European Patent Appl. No. 112,006. These documents also are entirely incorporated herein by reference. Materials in which some of the silicon atoms in the silicalite-1 or silicalite-2 structures are replaced by titanium atoms are known as titanium silicalites TS-1 and TS-2 and are described in DE 30 47 798 and J. S. Reddy, R. Kumar, and P. Ratnasamy, Appl. Catal., 58 (1990) L1-L4. These documents also are entirely incorporated herein by reference. Titanium silicalites are efficient catalysts for selective oxidation using hydrogen peroxide, in particular for the epoxidation of olefins (European Patent Appl. No. 100,119), the hydroxylation of aromatic compounds (DE 33 09 669 and J. S. Reddy, R. Kumar, and P. Ratnasamy, Appl. Catal., 58 (1990) L1-L4), the hydroxylation of aliphatic compounds (European Patent Appl. No. 412,596 and J. S. Reddy, S. Sivasanker, and P. Ratnasamy, J. Mol. Catal., 70 (1991) pp. 335-342) and the ammoximation of cyclohexanone (European Patent Appl. No. 208,311 and J. S. Reddy, S. Sivasanker, and P. Ratnasamy, J. Mol. Catal., 69 (1991) pp. 383-392). All of these documents are entirely incorporated herein by reference.
The known preparation of titanium silicalites TS-1 and TS-2 proceeds via a two-stage synthesis. First, a gel is produced by hydrolysis of a source of titanium, such as TiCl.sub.4, TiOCl.sub.2 or Ti(Oalkyl).sub.4, preferably Ti(Oalkyl).sub.4, and a source of silicon, such as silica gel or Si(Oalkyl).sub.4, preferably Si(Oalkyl).sub.4. Then this gel is crystallized in a hydrothermal synthesis by heating under pressure, wherein a template has to be added to promote crystallization, such as tetra-n-propylammonium hydroxide for TS-1 or tetra-n-butylammonium hydroxide for TS-2. The high price of Ti(Oalkyl).sub.4, Si(Oalkyl).sub.4 and the templates contribute greatly to the cost of producing TS-1 and TS-2.
The titanium silicalites TS-1 and TS-2 are mostly produced in the form of small crystallites with sizes of less than one micrometer in the known processes. These crystallites can only be separated from the liquid with difficulty by filtering. For industrial application of these materials, therefore, an additional agglomeration step is required. An example of such an agglomeration procedure is described in European Patent Appl. No. 203,260, which document is entirely incorporated herein by reference.
When using titanium silicalites TS1 and TS2 as catalysts for oxidation reactions using hydrogen peroxide, the catalytic activity is determined by the molecular size and molecular structure of the compound to be oxidized (M. Clerici and P. Ingallina, J. Catal., 140 (1993) pp. 71-83, which document is entirely incorporated herein by reference). This indicates that there is a restriction on the catalytic activity due to material transport inside the cavities in the crystal lattice, so that titanium atoms in the interior of the crystal contribute less to the catalytic activity than titanium atoms near the surface of the crystal.
There is a need, therefore, for catalysts which exhibit similar activity to titanium silicalites in selective oxidation reactions using hydrogen peroxide, and which can be prepared using small amounts of Ti(Oalkyl).sub.4, Si(Oalkyl).sub.4, and a template, and which enable targeted setting of the crystal size to enable better utilization of the catalytic activity of the titanium atoms.